Hydrogen is a “clean fuel” because it can be reacted with oxygen in hydrogen-consuming devices, such as a fuel cell or combustion engine, to produce energy and water. Virtually no other reaction byproducts are produced in the exhaust. As a result, the use of hydrogen as a fuel effectively solves many environmental problems associated with the use of petroleum based fuels. Safe and efficient storage of hydrogen gas is, therefore, essential for many applications that can use hydrogen. In particular, minimizing volume and weight of the hydrogen storage systems are important factors in mobile applications.
Several methods of storing hydrogen currently exist but are either inadequate or impractical for wide-spread consumer applications. For example, hydrogen can be stored in liquid form at very low temperatures. Cryogenic storage, however, only provides a volume density of 70 grams of hydrogen per liter, which is clearly insufficient for consumer applications. In addition, the energy consumed in liquefying hydrogen gas is about 60% of the energy available from the resulting hydrogen. Finally, liquid hydrogen is not safe or practical for most consumer applications.
An alternative is to store hydrogen under high pressure in cylinders. However, a 100 pound steel cylinder can only store about one pound of hydrogen at about 2200 psi, which translates into 1% by weight of hydrogen storage. More expensive composite cylinders with special compressors can store hydrogen at higher pressures of about 4,500 psi to achieve a more favorable storage ratio of about 4% by weight. Although even higher pressures are possible, safety factors and the high amount of energy consumed in achieving such high pressures have compelled a search for alternative hydrogen storage technologies that are both safe and efficient.
Other methods of hydrogen storage include the use of chemical compounds that either (i) chemically react with water or other species to generate hydrogen or (ii) reversibly adsorb and then release the hydrogen. However, these methods and compounds suffer from many deficiencies, which make them unsuitable for use in consumer applications. These deficiencies include, high cost, poor safety, poor hydrogen storage capacities, decreased reversibility, poor hydrogen generation capacities, poor control of hydrogen generation, and high system complexities.
In view of the above, there is a need for safer, more effective methods of storing and recovering hydrogen. In addition, there is a need to meet the above requirements while minimizing overall system volume and weight. It is, therefore, an object of the present invention to provide safer and more efficient methods for storing and generating hydrogen. These and other objects of the invention will become more apparent from the detailed description and examples that follow